The present invention relates to a method for measuring the wall thickness of a workpiece by ultra-sound, in which the surface echo of an ultrasonic wave generates a start pulse and the rear wall echo generates a stop pulse for a saw tooth generator and in which the amplitude of a saw tooth obtained when the stop pulse occurs provides a measure for the wall thickness.
For measuring the wall thickness of a workpiece by ultra-sound one records the surface echo and the rear wall echo obtained when irradiating ultrasonic waves perpendicularly upon the said surfaces, and measures the interval between the two echoes. This interval then provides a measure for the thickness of the workpiece. The recorded echoes cause corresponding triggering signals to be generated by the evaluation circuit.
The interval is measured by either of two methods. According to a first, namely an analog method of the type known from U.S. Pat. No. 3,427,866, the triggering signal produced by the surface echo starts a saw tooth generator which generates a signal rising linearly from zero. At the moment when the second triggering signal, the one produced by the rear wall echo, occurs one measures the instantaneous amplitude of the signal of the saw tooth generator. This analog method provides the disadvantage that the rising flank of the saw tooth must be very steep if high resolution is desired for the interval measurements. This however, considerably restricts the total measuring range.
According to the second, namely the digital evaluation method, one measure high-frequency counting pulses occurring between the two triggering signals. If in this case high resolution is desired for the interval measurements, the counting pulses must have a very high frequency. For example, if a resolution of 1 micron is desired in the case of steel, the counting pulses must have a frequency of approximately 3 GHz. Such high frequencies can be controlled only with a very considerable input in circuitry.
German Disclosure Document No. 31 26 138 describes a digital evaluation method of this type. However, in order to improve the resolution while reducing the counting frequency to every second triggering signal, the counting pulses are shifted in this case by a pre-determined phase angle. One then counts the number of counting pulses occurring between the two triggering signals over a given number of measuring cycles and determines thereafter the wall thickness by dividing the number of counting pulses by the number of measuring cycles.
According to the method known from German Disclosure Document No. 34 29 409, the residual time remaining after appearance of the second triggering pulse is recorded as a fraction of a counting pulse. The residual time is than also counted according to the Nenius principle by detuning an oscillator.